Potential roles of melatonin and ABA on apple dwarfing in semi-arid area of Xinjiang China

Abstract

Dwarfing is a typic breeding trait for mechanical strengthening and relatively high yield in modern apple orchards. Clarification of the mechanisms associated with dwarfing is important for use of molecular technology to breed apple. Herein, we identified four dwarfing apple germplasms in semi-arid area of Xinjiang, China. The internodal distance of these four germplasms were significantly shorter than non-dwarfing control. Their high melatonin (MT) contents are negatively associated with their malondialdehyde (MDA) levels and oxidative damage. In addition, among the detected hormones including auxin (IAA), gibberellin (GA), brassinolide (BR), zeatin-riboside (ZR), and abscisic acid (ABA), only ABA and ZR levels were in good correlation with the dwarfing phenotype. The qPCR results showed that the expression of melatonin synthetic enzyme genes MdASMT1 and MdSNAT5, ABA synthetic enzyme gene MdAAO3 and degradative gene MdCYP707A, ZR synthetic enzyme gene MdIPT5 all correlated well with the enhanced levels of MT, ABA and the reduced level of of ZR in the dwarfing germplasms. Furthermore, the significantly higher expression of ABA marker genes (MdRD22 and MdRD29) and the lower expression of ZR marker genes (MdRR1 and MdRR2) in all the four dwarf germplasms were consistent with the ABA and ZR levels. Considering the yearly long-term drought occurring in Xinjiang, China, it seems that dwarfing with high contents of MT and ABA may be a good strategy for these germplasms to survive against drought stress. This trait of dwarfing may also benefit apple production and breeding in this semi-arid area.

Keywords: Abscisic acid (ABA); Apple; Drought stress; Dwarfing; Melatonin (MT).

Figure 1. The phenotypes of the four dwarfing and control apple germplasms.

(A) Shoot lengths of the four dwarfing and control apple germplasms. Bar, 3 cm. (B) Internodal distance of the four dwarfing and control apple germplasms. (C) Paraffin sections of shoot of the four dwarfing and control apple germplasms. (D) Statistical chart of cell number per unit area. Data are means ± SD of triplicate studies. Different lower-case letters indicate significant differences among germplasms according to Fisher’s LSD test (p < 0.05).

Figure 2. The annual branches of dwarfing apple germplasms have higher MT and MDA levels than that of GB2.

(A) The melatonin level of the four dwarfing apple germplasms and control. (B) Relative expression level of MdASMT1 and MdSNAT5 in the four dwarfing and control apple germplasms. (C) The MDA content in the four dwarfing and control apple germplasms. Data are means ± SD of triplicate studies. Different lower-case letters indicate significant differences among germplasms according to Fisher’s LSD test (p < 0.05).

Figure 3. The ABA level is higher in dwarfing apple germplasms.

(A) The ABA content in the four dwarfing and control apple germplasms. (B) Relative expression level of MdCYP707A and MdAAO3 in the four dwarfing and control apple germplasms. (C) Relative expression level of MdRD22 and MdRD29 in the four dwarfing and control apple germplasms. Data are means ± SD of triplicate studies. Different lower-case letters indicate significant differences among germplasms according to Fisher’s LSD test (p < 0.05).

Figure 4. The ZR level is lower in dwarfing apple germplasms.

(A) The ZR content of the four dwarfing and control apple germplasms. (B) Relative expression level of MdIPT5, MdRR1 and MdRR2 in the four dwarfing and control apple germplasms. Data are means ± SD of triplicate studies. Different lower-case letters indicate significant differences among germplasms according to Fisher’s LSD test (p < 0.05).

Figure 5. Model of the molecular mechanism that mediates dwarfing of apple germplasms.

Enhanced content of melatonin, and ABA and reduced content of ZR induced by long-term drought stress resulted in the apple dwarfing phenotype. The red solid arrow indicates an increase and the blue solid arrow indicates a decrease. The black solid arrows denote direct activation.